You can also get them out of dumpsters on military installations. But there are some issues to be confronted here: (1) The filters that the masks use are sometimes specific to a certain agent(s). (2) Neither the filter nor the charcoal impregnation of the suit confers long-lasting immunity. The useful life ranges from minutes to days depending the what the suit and mask are exposed to. So there might be some surprised terrorist if they ever try to use one in a real situation!